1. Field of the Invention
The present invention relates to a laser mirror mount and, more particularly, to a mirror mount that provides passive pathlength control (PPLC) which compensates for the thermal expansion of the laser body.
2. Description of Related Art
Lasers have been known and used since the early 1960s. One class of laser utilizes two mirrors and a single linear path for generating a beam that may be used for various purposes. In another class of lasers, the laser path is established by three, four or more mirrors in a ring. Two or more laser beams are then propagated in opposite directions within the ring to establish a beat frequency. As the laser ring is moved, the beat frequency will change since the light beam traveling in the direction of movement will take slightly longer to traverse the ring, while the light traveling in the opposite direction of the movement will take slightly less time to traverse the ring. This forms the basis for a ring laser gyro which is used as a navigational device.
Typical ring laser gyros may utilize two, four, six or more frequencies. As stated, they may also utilize three, four or more mirrors to form the desired ring.
Ring laser gyros may be formed as a flat laser ring or a folded laser ring. A typical flat laser ring utilizing four mirrors is shown in U.S. Pat. No. 4,317,089, patented Feb. 23, 1982 by D. C. Grant, et al. A ring laser gyro formed by four mirrors arranged on the corners of a block that defines the vertices of a tetrahedron is shown in U.S. Pat. No. 4,813,774, patented Mar. 21, 1989, by T. A. Dorschner, et al., which is assigned to the same assignee as the present invention. A similar laser formed on a block with four mirrors forming a ring on two orthogonal planes and only two of the four mirrors on the vertices of the block is shown in U.S. Pat. No. 4,818,087, issued Apr. 4, 1989, to T. A. Dorschner, also assigned to the assignee of the present invention. A low-cost, six-frequency ring laser gyro with a non-planar, irregular closed path between mirrors is shown in U.S. Pat. No. 4,687,331, patented Aug. 18, 1987, to M. Holz, et al., assigned to the same assignee at this invention.
Almost all present day ring laser gyros use one or more actively controlled, deformable mirrors to maintain a constant pathlength therein. Control of the deformable mirrors is generally accomplished by use of a thin diaphragm which is flexed by a piezoelectric crystal. It has been found that this flexure of ring laser gyro mirrors using piezoelectric crystals causes the diaphragmed mirrors to tilt as flexed. The flexure can also occur with temperature changes of the pathlength control mirror. The mirror tilt due to flexure or temperature change causes the reflection point upon the mirror that establishes the laser pathlength to walk across the mirror. Such beam walk causes significant changes within the pathlength of the laser which can cause the output of the gyro to vary several degrees per hour. The laser bias caused by those variations is an error all manufacturers strive to reduce.